Technical Field
The present invention relates to a laser radar apparatus that detects the presence, orientation, distance, and the like of an object using laser light.
Background Art
A laser radar apparatus is used as an apparatus that uses laser light to detect the presence and orientation, as well as distance and the like, of an object in a detection area to be targeted. In general, in the laser radar apparatus, laser light (outgoing light) emitted from a laser light emitting unit is reflected towards the direction of the detection area by a rotating mirror and is scanned in accompaniment with the rotation of the rotating mirror. In addition, laser light (returning light) that has been reflected to an object and returned is reflected towards a detecting unit by the rotating mirror. The detecting unit detects information related by the object by receiving the reflected light. The outgoing light is set to have a divergence angle that is small and to be substantially parallel, so that distant objects can be detected.
In the laser radar apparatus, the presence and orientation of an object is detected based on whether or not an intensity level of reflected light is an object-present level. The distance to the object is measured by an amount of time from emission of the laser light to reception of the reflected light. In addition, in the laser radar apparatus, when the object is far, regardless of the small divergence angle of the laser light, the laser light spreads to a certain extent while traveling to-and-fro over the long distance. Therefore, the intensity of the reflected light reflected by the distant object becomes low. Meanwhile, when the object is near, the laser light is reflected with little spreading. Therefore, in a laser radar apparatus that is configured such that almost all of the reflected light enters a detecting unit (such as a laser radar apparatus that is configured such that an emission optical axis and a reflection optical axis of the laser light differ), the intensity of the reflected light becomes significantly high.
Therefore, the detecting unit outputs an electrical signal of a level that is based on the intensity (light reception intensity) of the reflected light. However, to enable detection of objects having the same reflectance over long distances (several tens of meters) to short distances (several centimeters), uniformly, at substantially the same electrical signal level, the detecting unit performs correction, such as attenuating the electrical signal for short distances (determined by measured time) and raising a threshold for detection of electrical signals. As a result, fog and the like that have low reflectivity are not detected.
Meanwhile, in the laser radar apparatus, a laser radar apparatus of a type in which detection capability is improved by the emission optical axis and the reflection optical axis of the laser light being matched is provided. A basic configuration of this type of laser radar apparatus is shown in FIG. 1. A laser radar apparatus 51 in FIG. 1 includes a laser light emitting unit 52 that emits laser light; a reflection mirror for emission 53, a rotating mirror 54 that is used for both scanning of outgoing light and light reception, a reflection mirror for light reception 55 in which a hole 55a is formed, a detecting unit 56 that detects reflected light reflected by an object, and a light reception lens 57. A reflective surface 54a of the rotating mirror 54 is formed into a flat surface.
The laser light emitted from the laser light emitting unit 52 is emitted in a form similar to parallel light of which the divergence angle is suppressed, so that an object at a far distance can be detected.
In the laser radar apparatus 51, outgoing light 52a emitted from the laser light emitting unit 52 is reflected by the reflection mirror for emission 53. The outgoing light 52a passes through the hole 55a in the reflection mirror for light reception 55 and is reflected by the flat reflective surface 54a of the rotating mirror 54. The outgoing light 52a is emitted towards the direction of a detection area. Reflected light 52b that is reflected by an object and incident on the rotating mirror 54 is reflected towards the direction of the reflection mirror for light reception 55 by the rotating mirror 54. Subsequently, the reflected light 52b is reflected towards the direction of the light reception lens 57 by the reflection mirror for light reception 55, and detected by the detecting unit 56.
In this case, an emission optical axis La on which the outgoing light 52a passes through the reflection mirror for emission 53 and travels towards the rotating mirror 54, and a reflection optical axis Lb on which the reflected light 52b is reflected by the rotating mirror 54 travels towards the reflection mirror for light reception 55 coincide. Structurally, a portion of the reflected light 52b passes through the hole 55a of the light reception reflection mirror 55 towards the direction of the reflection mirror for emission 53.
The laser radar apparatus 51 is required to perform detection over distances from far, at several tens of meters, to near, at several centimeters. Taking into consideration detection of distant objects, in particular, the laser light is emitted in a form in which the divergence angle is small, such that the degree of attenuation of the laser light does not become large.
Here, a relationship between a light reception amount detected by the detecting unit 56 and the distance from the laser radar apparatus 51 to an object is shown in FIG. 2. As the separation distance from the laser radar apparatus 51 to an object, for example, point P1, point P2, point P3, point P4, and point P5 are indicated, from the farthest point in this order. As shown in FIG. 3(a), regarding the reflected light 52b reflected by an object at point P1 that is far from the laser radar apparatus 51, the diameter of the reflected light 52b becomes greater than the reflective surface 54a of the rotating mirror 54 (i.e. it diffuses). Therefore, the intensity itself of the reflected light 52b is weak. Furthermore, because the diameter of the reflected light 52b is large, although a portion of the reflected light can be reflected by the overall area of the reflective surface 54a, the remaining reflected light 52b passes through to the periphery of the reflective surface 54a. Therefore, light reception loss is large. As a result, the light reception amount of the detecting unit 56 is low.
Because point P2 is closer to the laser radar apparatus 51 than point P1, as shown in FIG. 3(b), the intensity itself of the reflected light 52b reflected by the object is slightly strong, and relative light reception loss is small. In this case, the light reception amount at the detecting unit 56 also increases. In addition, at point P3, as shown in FIG. 3(c), the diameter of the reflected light 52b becomes substantially the same as the size of the reflective surface 54a of the rotating mirror 54. The light reception amount becomes maximum. In the cases of points P1, P2, and P3, above, a portion (near the optical axis) of the reflected light 52b received by the rotating mirror 54 passes through from the hole 55a in the reflection mirror for light reception 55.
Next, because point P4 is even closer to the laser radar apparatus 51, the reflected light 52b reflected by the object has a high light intensity. However, because the diameter of the reflected light 52b is smaller than the reflective surface 54a, as shown in FIG. 4, reflection area of the reflected light 52b that is reflected towards the direction of the detecting unit 56 by the reflection mirror for light reception 55 becomes small. That is, the proportion of the reflected light 52b passing through the hole 55a increases. As a result, the light reception amount at the detecting unit 56 decreases.
Next, because point P5 is even closer to the laser radar apparatus 51, as shown in FIG. 5, the reflected light 52b reflected by the object is such that the diameter of the reflected light 52b becomes even smaller than the reflective surface 52a. As a result, the overall light reception amount at the detecting unit 56 further decreases. In PTL 1, a laser radar apparatus in which an emission optical axis and a reflection optical axis are the same is configured such that an overall area of a reflection area of outgoing light on a rotating mirror is formed by a flat, planar reflective portion.